This invention relates to electric motor control, and in particular to pulse width modulation (PWM) control of multiple phase brushless motors.
WO 2004/023639 describes a low-acoustic-noise low-speed position sensorless algorithm which uses measurements of the instantaneous rate of change of phase current (di/dt) within a PWM period to estimate rotor position down to zero speed. The algorithm shifts the edges of the PWM pattern to ensure that the necessary inverter states are available for the required amount of time to measure the rate of change of current in the required phase and under the required voltage excitation condition. By shifting the PWM edges in this way the acoustic noise created by the sensorless algorithm is kept to a minimum. The di/dt signal can be measured in any one of the eight inverter states.
WO 2004/023639 and PCT/IB2009/051914 describe several different ways to combine di/dt measurements from different inverter states to provide an estimate of position. Different combinations have different advantages such as immunity to temperature variation, magnitude of available voltage for controlling motor torque, acoustic noise, position estimate accuracy and so on. The two patents mentioned only use combinations of di/dt measurements using some or all of the six non-zero inverter states. However, the academic literature also describes techniques that use at least on of the two zero inverter states.